2020MNRAS.493..250P

We present the detection and analysis of a weak low-ionization absorber at z = 0.121 22 along the sightline of the blazar PG 1424+240, using spectroscopic data from both HST/COS and STIS. The absorber is a weak Mg II analogue, with an incidence of weak C II and Si II, along with multicomponent C IV and O VI. The low ions are tracing a dense (nH ∼ 10-3 cm-3) parsec-scale cloud of solar or higher metallicity. The kinematically coincident higher ions are either from a more diffuse (nH ∼ 10-5-10-4 cm-3) photoionized phase of kiloparsec-scale dimensions or are tracing a warm (T ∼ 2 x 105 K) collisionally ionized transition temperature plasma layer. The absorber resides in a galaxy overdense region, with 18 luminous (>L*) galaxies within a projected radius of 5 Mpc and velocity of 750 km s-1. The multiphase properties, high metallicity, and proximity to a 1.4L* galaxy, at ρ ∼ 200 kpc and separation |[?]v| = 11 km s-1, favour the possibility of the absorption tracing circumgalactic gas. The absorber serves as an example of weak Mg II-O VI systems as a means to study multiphase high-velocity clouds in external galaxies.